1. Field of the Invention
This invention pertains to piezoelectric elements, and particularly to circuits and methods for driving piezoelectric elements utilized in such devices such as pumps, for example.
2. Related Art and Other Considerations
A piezoelectric element is a crystalline material which produces an electric voltage when subjected to mechanical pressure. In view of their various properties, piezoelectric elements have been used as actuators in diaphragm displacement pumps. In general, piezoelectric actuators of the type used in pumps require excitation by a regularly reversing high-voltage field. Depending on the application, the excitation voltage may be anywhere from 25 to 1000 volts or more and the frequency of field reversal may be anywhere from a fraction of a cycle per second to thousands of cycles per second. Typically, this excitation signal must be derived from a relatively low-voltage source of 1.5–25 volts. It is desirable that this derivation or conversion be very energy efficient and that the associated components be inexpensive.
In addition, given that both the piezoelectric actuators and the devices that employ them often have many resonant characteristics, it is desirable for the field reversal to be monotonic—e.g., a sine wave.
An example of a reasonably effective drive circuit for driving piezoelectric elements used as pump actuators is disclosed in U.S. patent application Ser. No. 10/380,547 and U.S. patent application Ser. No. 10/380,589 (both filed Mar. 17, 2003, both entitled “Piezoelectric Actuator and Pump Using Same”, and both incorporated by reference herein in their entirety). That drive circuit comprises a EL lamp driver circuit which was originally designed to drive electro-luminescent (EL) lamps, but which has now ingeniously been employed in the referenced documents for driving piezoelectric pumps. The EL lamp driver circuit is a high-powered, switch-mode integrated circuit (IC) inverter intended for backlighting color LCDs and automotive applications. The specially designed EL lamp driver IC and a few components such as a discharge circuit comprise a complete EL lamp driving circuit.
Described in more detail, the EL lamp driver circuit uses a relatively high frequency oscillator or state-machine to drive a flyback circuit to produce high-voltage charges that are stored in a storage capacitor. The storage capacitor is then treated as a high-voltage source of direct current which is applied to a bridge-type switching circuit that is driven by either a second oscillator or state-machine or a signal derived from the flyback oscillator to produce a reversing field effect.
These EL lamp driver circuits have been widely employed in the electroluminescent lighting industry and consequently many of the circuit elements have been integrated into “one-chip” solutions. This EL lamp driver technology/circuitry has evolved to drive the displays of handheld electronic devices such as cell phones, Personal Digital Assistants (PDAs) and electronic games. The circuits can operate at low frequency and current draw, and at relatively high frequencies, making them very attractive for portable applications. Moreover, equipped with a discharge circuit design, the EL circuit minimizes EL lamp system noise, i.e., noise that would affect the operation of other IC's or chips.
Despite its overall ingenious and overall beneficial utilization in piezoelectric pumps, some aspects of using a EL lamp driver circuit are problematic. Several example problems are now briefly described.
As a first example problem, the EL lamp driver is limited in that it operates only at a fixed frequency once installed. The oscillators and/or state machines used in EL lamp drivers are fixed. Therefore, the EL lamp driver circuits are “Mona Lisa's”—each circuit having a fixed flyback frequency. As a result, when used in a piezoelectric pump, the EL lamp driver circuit provides a fixed piezoelectric drive frequency and a fixed output voltage to input voltage/load ratio. When used in a piezoelectric pump, the EL lamp driver circuits are “tuned” to a specific piezoelectric application by varying component values at the time of manufacture.
As a second example problem, the output wave form of the EL lamp driver circuit is a modified sawtooth which creates audible noise in the piezo even under load (due to the sharp peak on the waveform output by the EL lamp driver circuit). This is due, in part, to the architecture of the EL lamp driver circuit which employs a crude, somewhat direct current source. This current source is digitally switched by a bridge circuit to produce the reversing field, so that the resulting drive waveform is far from pure. Square waves and sawtooths are common with ragged, time-varying frequency content signals being typical. But in non-audio applications piezoelectric actuators and the devices that employ them typically need to operate at pure frequencies for maximum efficiency and to produce the least amount of audible noise. So when a drive waveform is applied that has frequency content outside of the targeted fundamental drive frequency, that extraneous frequency content adds little to the work output of the piezoelectric element but greatly increases undesirable actuator audible noise.
As a third example problem, the only variable user input to the EL lamp driver circuit is the voltage input (Vin). The EL lamp driver architecture employs a unipolar voltage source to drive the piezoelectric actuator in bipolar fashion. Given this fact, it is unavoidable that both “sides” of the piezoelectric actuator are subjected to voltage potentials other than system ground. In applications such as a pump which pumps a conductive liquid, it is highly desirable that the fluid side of the actuator always remain at system ground. This cannot be achieved using the EL lamp driver circuitry.
The current drive approach does not have a means of accepting external control inputs or monitoring local actuator related parameters. Capabilities such as resonance detection, pressure feedback, temperature feedback, external modulation, etc. are not even considered. The lack of capability to vary the frequency or voltage on the board once installed severely limits the capability of the circuit when trying to optimize the frequency or voltage to address back pressures, temperatures and other operating conditions.